The present invention relates to a method of measuring the error rate of an optical transmission system and apparatus for implementing the method.
Optical networks are increasingly used nowadays in high bit rate transmission systems. Optical networks provide functions, such as switching, that are transparent, i.e. independent of the electrical signal transmitted, and thus offer the flexibility required in modern telecommunication networks.
However, this transparency necessitates verifying that the signal transmitted conforms to what is required, in particular in terms of transmission quality. It is therefore essential to have transparent means for determining the quality of the signal transmitted independently of the format of the signal, in particular its transmission bit rate and its type of modulation, in order to be able to measure the quality of optical transmission over any type of network (backbone network, MAN, LAN, etc.) regardless of its data format (SONET, SDH, IP over WDM, Giga-Ethernet, etc.) and its bit rate (622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s, etc.).
There are many causes of signal degradation in optical networks. They include amplified spontaneous emission (ASE) by amplifiers, chromatic dispersion generating inter-symbol interference (ISI), out-band crosstalk (linked to an adjacent channel) and in-band crosstalk (caused by an interfering wave at the same wavelength as that measured). These causes of degradation are additional to non-linear effects such as the Kerr, Brillouin, and Raman effects.
The principal quality criterion of a digital optical network is its bit error rate (BER), which is defined as the probability of the receiver detecting an erroneous bit. Because of noise, the signal received at the receiver fluctuates around an average value I1 (if a 1 was transmitted) or I0 (if a 0 was transmitted). It is assumed that the distribution is Gaussian in both cases. The distribution of the 1 level therefore has as its parameters I1 and the variance "sgr"21, while the distribution of the 0 level has as its parameters I0 and the variance "sgr"20. To decide if a value received by the receiver is correct, it is necessary to impose a decision threshold ID. A bit sent at 1 is considered to be correct if I greater than ID and a bit sent at 0 is considered to be correct if I less than ID In other words, an error has occurred if I less than ID for a bit sent at 1 or if I greater than ID for a bit sent at 0. In practice, ID is optimized to minimize the BER.
The BER is defined by the equation:   BER  =            exp      ⁢              (                              -                          Q              2                                /          2                )                    Q      ·                        2          ·          π                    
in which Q, referred to as the quality factor, is defined by the equation:   Q  =                    I        1            -              I        0                            σ        0            -              σ        1            
A method of determining the quality of an optical signal independently of the format of the signal by using relative error rate measurements is already available.
This method, described in the document xe2x80x9cField Trial over 750 km long transparent WDM link using an adaptive 10 Gb/s receiver with non-intrusive monitoring capabilityxe2x80x9d, S. Herbst et al., OFC 2001 (paper ML2-1), for example, is based on measuring the amplitude of the detected electrical signal by using an exclusive-OR function to compare the decisions of two bistables, one operating at the optimum threshold ID (optimum amplitude from which the signal is considered to be equal to 1) and the other operating with a variable amplitude threshold. The difference between the signals from the two bistables, referred as the pseudo-error, is logged each time that the two measurements are different. Assuming a Gaussian distribution of the levels, extrapolating the pseudo-error rate curves as a function of the position of the variable amplitude threshold provides an evaluation of the BER at the optimum threshold.
The above method is intrinsically transparent to the format of the signal transmitted. However, it necessitates the use of a clock recovery circuit and a variable delay line for phase adjustment. These components introduce a non-negligible cost factor and additionally limit the transparency of the method because they cannot be tuned over a wide range of signal bit rates.
Another method of solving this problem, known as the histogram method, is also available. This method applies asynchronous sampling to the transmitted signal, so that the sampling is independent of the bit rate of the signal, after which all of the samples are placed on the amplitude axis. A histogram representing the number of samples as a function of amplitude is then extracted. Then, after eliminating problematical points using a heuristic method, an estimate is derived from the histogram using two Gaussian distributions to determine the Q factor and then the BER.
That method is not always satisfactory. It provides only a qualitative evaluation of the error rate, because the results that it supplies are not reliable.
An object of the present invention is therefore to provide a method of measuring the error rate of an optical transmission system that is transparent not only to the format of the transmitted signal but also to the signal transmission bit rate, and which necessitates the use of components that are less costly than the prior art method.
To this end, the present invention proposes a method of measuring the error rate of an optical transmission system transmitting a signal, said method comprising the following operations:
detecting said signal,
asynchronously sampling said signal at a frequency independent of the bit rate of said signal to obtain K samples of said signal at respective times t1 to tK where K is an integer greater than or equal to 2,
computing the eye diagram of said signal, and
computing the error rate of said signal,
which method is characterized in that, after sampling said signal, it further comprises an operation of computing the bit time of said signal.
The method of the invention solves the problem caused by the prior art methods using asynchronous sampling, namely the inaccuracy of the result. The method of the invention computes the bit time so that the same advantages are obtained as with a synchronous method using a physical clock recovery system, but the clock recovery system is no longer necessary. The method of the invention is also transparent to the transmission bit rate.
By means of the invention, the eye diagram can be reconstructed without knowing the bit rate, i.e. without knowing the real bit time of the optical signal, because it is computed from the signal sampled asynchronously.
Furthermore, asynchronous sampling of the received optical signal guarantees that the method of the invention is transparent to the type of modulation. The asynchronous sampling can be carried out at a frequency very much lower than the bit rates used, which means that it is not synchronized to the signal.
Computing the bit time of the signal is an essential step for reconstituting the eye diagram when the sampling is asynchronous.
Note that, in the context of the invention, the expression xe2x80x9cbit timexe2x80x9d is used both for the absolute bit time and for the bit time relative to the sampling frequency.
In a first implementation of the method of the invention, the absolute bit time is computed from an approximate value To known initially.
To this end, simultaneous computation of the bit time and the eye diagram comprises the following operations:
choosing a sub-sample of K/N samples of said signal where N is an integer power of 2,
separating said sub-sample into two parts,
computing two eye diagrams from the respective parts of the sub-sample using the value T0 for the bit time,
computing two histograms from the two eye diagrams by digitizing the time and the intensity,
determining the time period xcex4 between the two histograms,
determining the bit time T1 from the equation:             T      1        =                  T        0            -              2        ⁢                  xe2x80x83                ⁢        δ        ⁢                              T            0                                t            k                                ,  and
repeating the above operations substituting N/2 for N until a sub-sample of K/2 samples is obtained.
This implementation is particularly simple and necessitates only a very approximate initial knowledge of the bit time; furthermore, it computes very accurately the real bit time, which differs greatly from the bit time known initially because of the inaccuracy relating to the signal clock. This implementation also reconstitutes the eye diagram.
In a second implementation of the method of the invention, the bit time relative to the sampling frequency is computed without initially knowing the bit time.
To this end, the following operations are effected:
applying a non-linear function to the series of samples of the signal to obtain a series of substantially periodic values yk for k varying from 1 to K,
dividing the series into M sub-series each of L elements where L and M are integers,
computing the discrete Fourier transform of each sub-series, which yields a function Yi for i varying from 1 to M,
defining a periodogram function as the ratio with respect to M of the sum of the squares of the moduli of the functions Yi for i varying from 1 to M, and
determining the frequency f which maximizes the periodogram function.
To compute the eye diagram, the following operations are then effected:
computing the discrete Fourier transform at the frequency f of the series yk, which yields a function zk for k varying from 1 to K, and
obtaining the time associated with each sample of the signal from the equation:       τ    k    =            arg      ⁡              (                  Z          k                )                    2      ⁢              xe2x80x83            ⁢      π      
In an advantageous implementation, the Fourier transform is computed over a sliding window centered on yk. This avoids errors due to cumulative phase jitter affecting the sampling clock or the signal.
This second implementation dispenses completely with the need for any initial knowledge of the bit time, and it improves tolerance to jitter affecting the sampling clock or the signal.
In the invention, when the bit time has been determined and the eye diagram reconstituted by either of the above methods, the error rate is computed by modeling the statistical distributions of the levels of the signal by means of P Gaussian distributions where P is an integer greater than or equal to 2 and preferably equal to 8. This takes better account of the deterministic levels resulting from inter-symbol interference.
Finally, the invention also provides apparatus for implementing the above method, which apparatus comprises:
means for detecting the signal,
means for sampling the detected signal at a frequency independent of the bit rate of the signal,
means for digitizing samples obtained at the output of the sampling means, and
software for processing the digitized samples to compute the bit time and the eye diagram.
The software can also model the statistical distributions of the levels in order to compute the error rate of the signal.
Other features and advantages of the present invention become apparent in the course of the following description of an embodiment of the invention, which is provided by way of illustrative and non-limiting example.